High power amplifier

ABSTRACT

A high power amplifier architecture is disclosure. One example configuration includes a first plurality of distributed amplification stages operatively coupled in a first string. A conductive trace associated with the first string provides a stepped structure, such that the associated inductance successively decreases from input to output of the first string. A second plurality of distributed amplification stages is operatively coupled in a second string, and a conductive trace associated therewith provides a stepped structure, such that the associated inductance successively decreases from input to output of the second string. In one example case, each of the first and second strings comprises gallium nitride transistor amplification stages formed on silicon carbide. The module may further include a heat spreader material that thermally and electrically couples to the amplification stages. The conductive trace associated with one string can be shared with another string.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalApplication No. 61/299,072, filed Jan. 28, 2010, and is acontinuation-in-part of U.S. application Ser. No. 11/629,025, filed Dec.8, 2006, which is a National Stage of International Application No.PCT/US05/39407, filed Nov. 1, 2005, which claims the benefit of andpriority to U.S. Provisional Application No. 60/630,343, filed Nov. 23,2004. Each of these applications is herein incorporated by reference inits entirety.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with United States Government support. Thecontract is classified, as is the awarding agency. The contract's publicreference number is C-8385. The United States Government has certainrights in this invention.

FIELD OF THE DISCLOSURE

This invention relates to high power amplifiers, and more particularly,to compact high power radio frequency transmitters using wide band gapsemiconductor technology, wideband distributed circuit architecture, andthermal packaging approaches.

BACKGROUND

Many existing Electronic Warfare (EW) systems have a requirement for ahigh Radio Frequency (RF) power transmitter (ranging from tens-of-wattsto several hundred watts) having performance over a wide instantaneousfrequency bandwidth. Currently, these EW systems are completely relianton the use of Traveling Wave Tube Amplifier (TWTA) technology to achievethe combination of wide frequency bandwidth and high output powerperformance. Tube based amplifiers, however, may have two very limitingdisadvantages: (1) they are relatively large in physical size and (2)they require very high-voltage power supplies (sometimes thousands ofvolts). These characteristics may limit the use of TWTAs in transmitterapplications that cannot support the large size or high-voltage powersupply constraints of the tube-based amplifiers.

Transmitter applications may require some form of High Power Amplifier(HPA) to achieve the RF output power specified for a particulartransmitter system. Only two approaches are currently available: (1)tube-based amplifier technology or (2) a solid-state solution. Theapproaches tradeoff output power level for size and reliability. Intube-based solutions, the high power density capability of the TWTAprovides superior output power performance. However, the high-voltagepower supply requirement for these amplifiers is not suitable forproviding a compact form-factor solution. Present wideband solid-stateamplifiers, using established gallium arsenide (GaAs) transistortechnology, have significantly lower power density and are primarilylimited to low to medium RF power applications (usually less than 15 W).Their power density capability requires a significant level of circuitpower-combining, which usually limits the obtainable RF output powerlevel.

The GaAs power solid-state amplifiers do, however, offer a smaller sizethan tube-based approaches. More recently, Microwave Power Modules (MPM)have offered the combination of a solid-state driver amplifier whichthen feeds a TWTA as the power stage. This combination has offered asmaller form-factor than the tube alone, but a high voltage power supplyis still required. In addition, tube-based amplifiers have beenassociated with reliability concerns and may not be an option in EWsystems that cannot accommodate the size and high-voltage power supplyconstraints of TWTAs. In general, for wideband, high power requirements,the cumbersome TWTA, along with their high-voltage power supplyrequirements, do not support transmitter systems requiring smallform-factors.

Thus, traditional methods of achieving high power transmitters operatingover wide bandwidths utilize large form-factor TWTAs. The use of thesetube-based approaches does not permit small form-factor amplifierpackages due to the large physical size of the tubes and theirassociated high-voltage power supplies. Therefore, there is a need forsystems that provide small, wideband, high power solid-statetransmitters.

SUMMARY

One embodiment of the present invention provides a high power amplifiermodule. The module includes a first plurality of distributedamplification stages operatively coupled in a first string, wherein aconductive trace associated with the first string is a steppedstructure, such that the associated inductance successively decreasesfrom input to output of the first string. The module further includes asecond plurality of distributed amplification stages operatively coupledin a second string, wherein a conductive trace associated with thesecond string is a stepped structure, such that the associatedinductance successively decreases from input to output of the secondstring. In one specific example case, each of the first and secondstrings comprises gallium nitride transistor amplification stagesinterconnected by inductors, and wherein the values of said inductorsare set such that the voltage and current associated with one of thegallium nitride transistor stages is equal to that associated with theother gallium nitride transistor stages to facilitate maximum powertransfer and matching between interconnected stages. In one suchspecific case, the inductors have values that are set such that theimpedance associated with one of the gallium nitride transistor gainstage is not equal to the impedance associated with an adjacent galliumnitride transistor gain stage. In another such specific case, theinductors have values that are set such that the impedance associatedwith two of the gallium nitride transistor stages is not equal to theimpedance associated with an adjacent two gallium nitride transistorstages. In another specific example case, each of the first and secondstrings comprises gallium nitride transistor amplification stages formedon a silicon carbide substrate. In another specific example case, theamplifier module further includes a heat spreader material thatthermally and electrically couples to the gallium nitride transistorstages. In one such case, the heat spreader material is diamond with ametallic coating. The metallic coating can be, for example, gold, insome cases. In another such case, the heat spreader material is chemicalvapor deposited diamond with a metallic coating. In one such case, themetallic coating encapsulates the chemical vapor deposited diamond. Inanother specific example case, the conductive trace associated with thefirst string is shared with a third string comprising a third pluralityof distributed amplification stages. In another specific example case,the high power amplifier module has a solid-state package design toaccommodate greater than 8 W/mm² of power dissipation. In anotherspecific example case, the high power amplifier module has a nominalpower gain of greater than 45 dB, a nominal output power of greater thanabout 50 W, and a frequency bandwidth performance from 1-8 GHz. In onesuch case, the high power amplifier module has a package dimension equalor less than 1.2 cubic inches. In another specific example case, thehigh power amplifier module further comprises at least one of apre-driver and a driver. Numerous variations on this power amplifierarchitecture will be apparent in light of this disclosure.

For example, another embodiment of the present invention provides a highpower amplifier. This example configuration includes a wideband,monolithic microwave integrated circuit comprising modular buildingblocks including a non-uniform distributed amplifier string comprised ofa number of gallium nitride transistor amplification stagesinterconnected by inductors. The module further includes a heat spreadermaterial that thermally and electrically couples to the gallium nitridetransistor stages. In one example such case, the gallium nitridetransistor is a dual field plate transistor with a silicon carbidesubstrate. In another such case, the values of the inductors are setsuch that the voltage and current associated with one of the galliumnitride transistor stages is equal to that associated with the othergallium nitride transistor stages to facilitate maximum power transferand matching between interconnected stages. In another such case, theheat spreader material is chemical vapor deposited diamond with ametallic coating.

Another embodiment of the present invention provides a method forforming a power amplifier. The method includes operatively coupling afirst plurality of distributed amplification stages in a first string,and providing a conductive trace associated with the first string thatis a stepped structure, such that the associated inductance successivelydecreases from input to output of the first string. The method furtherincludes operatively coupling a second plurality of distributedamplification stages in a second string, and providing a conductivetrace associated with the second string that is a stepped structure,such that the associated inductance successively decreases from input tooutput of the second string. The method further includes providing aheat spreader material that thermally and electrically couples to thegallium nitride transistor stages. In one such case, the conductivetrace associated with the first string is shared with a third stringcomprising a third plurality of distributed amplification stages.

The features and advantages described herein are not all-inclusive and,in particular, many additional features and advantages will be apparentto one of ordinary skill in the art in view of the drawings,specification, and claims. Moreover, it should be noted that thelanguage used in the specification has been principally selected forreadability and instructional purposes, and not to limit the scope ofthe inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of frequency (GHz) versus gain (dB) and shows theMeasured Small-signal Gain (MSG/MAG) of 0.25 μm, 0.35 μm, and 0.5 μmgate-length dual field plate GaN, High Electron Mobility Transistor(HEMT) devices using 800 μm of total gate periphery, in accordance withan embodiment of the present invention.

FIG. 2A shows a block diagram of a GaN based high power amplifier module200 configured in accordance with an embodiment of the presentinvention.

FIG. 2B shows a cross-sectional diagram of a GaN based high poweramplifier module 200 configured in accordance with an embodiment of thepresent invention.

FIGS. 3A-C show a series of layout diagrams of GaN HPA MMICs utilizingan example NDPA circuit architecture, including the pre-driver, driver,and HPA designs, in accordance with an embodiment of the presentinvention.

FIG. 4 is a plot of frequency (GHz) versus magnitude (dB) ofsmall-signal performance of twelve GaN driver MMICs shown in FIG. 3B, inaccordance with an embodiment of the present invention.

FIG. 5 is a diagram of an example non-shared drain HPA 500, configuredin accordance with an embodiment of the present invention.

FIG. 6 is a plot of the non-shared drain HPA 500 performance showingboth frequency (GHz) versus power gain (dB) and frequency (GHz) versusthe input/output power ratio (dBm), in accordance with an embodiment ofthe present invention.

FIG. 7 is a diagram of a GaN HPA module 700 configured in accordancewith an embodiment of the present invention.

FIG. 8A shows the output power of GaN amplifier module 700 at Vd=35 Vand Vg=−1.9V for a 10% duty cycle; 10 μsec pulse width, in accordancewith an embodiment of the present invention.

FIG. 8B shows the output power of GaN amplifier module 700 at Vd=35 Vand Vg=−1.9V for 2 μsec pulse width and 1 msec period at 2, 3, and 4GHz, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Some embodiments of the present invention may incorporate three elementsinto a solution for achieving a small form-factor solid-state High PowerAmplifier (HPA). Such embodiments may produce smaller sized, high powerEW transmitters not currently achievable with tube-based amplifiertechnology. In one specific example configuration, the first of theseelements employs the use of gallium nitride (GaN), a high-power-densitysemiconductor technology, capable of nearly a 10-fold increase in RFdevice output power over currently available gallium arsenic (GaAs)semiconductor technology. Dual field plate GaN transistors represent awideband gap semiconductor technology that is suitable for operation upthrough 20 GHz and capable of providing a sufficiently high powerdensity to achieve RF power levels comparable to those of TWTAs. GaNtransistors operate at significantly lower voltages than TWTAs (e.g., 25to 40 V), but are capable of achieving power densities that enable GaNtransistors, appropriately combined, to provide comparable output powerlevels to those obtained from TWTAs. Some embodiments of the presentinvention that employ GaN transistors as TWTA replacements furtherprovide sufficient thermal management in the package design toaccommodate a high (e.g., 8-12 W/mm²) power dissipation, and utilize acircuit architecture that can provide a wideband frequency performanceand maintain a high-output power level.

One example embodiment of the present invention integrates: (a) advancedGaN device technology, (b) wideband, distributed circuit architecturewith Non-Uniformly Distributed Power Amplifier (NDPA), and (c) a thermalpackaging approach employing advanced thermal management. Thecombination of these elements offers a solution for the problem ofachieving a small form-factor, solid-state replacement for widebandTWTAs.

In one specific such embodiment, the first element involves the use of adual field plate GaN device on a silicon carbide (SiC) substrate whichprovides a transistor technology that operates with sufficiently highpower density to achieve output power levels comparable to TWTAs.Currently available GaAs technology can provide RF power densities ofonly 1-2 W/mm. Achieving output power levels of, for instance, 50 to 100watts, requires in excess of 25 to 50 mm of total device periphery, anon-realistic size for GaAs Monolithic Microwave Integrated Circuits(MMIC) technology. The use of GaN technology, however, can provide, forinstance, greater than 5 W/mm of output power density for realistic diearea (e.g., 14-20 mm of device periphery). Hence, high levels of outputpower can be achieved using realizable die sizes that can offercomparable power levels of tube-based amplifiers.

The second element of this specific example embodiment involves the useof a circuit architecture that is capable of multiple octaves offrequency performance critical for the wideband nature of many EWtransmitter systems. By employing NDPA circuit architecture in MMICsthat integrate GaN based transistors, the required bandwidth performancetypical of TWTAs can be achieved. The use of the NDPA architecture isnot only advantageous for its wideband frequency performance, but thedistributed nature of the NDPA topology facilitates the thermalmanagement on the GaN die. The individual transistor heat-sources aremore distributed than other more conventional MMIC amplifierarchitectures, and therefore, proximity heating of individualtransistors contained on the MMIC is minimized.

The third element of this specific example embodiment includes a thermalmanagement technology that is employed in the amplifier package andallows high-power-density devices to be integrated into miniaturizedpackages, achieving acceptable electrical performance without the needfor liquid or phase-change cooling approaches, in accordance with somesuch embodiments. A high thermal conductivity heat-spreader material,such as a metallic coated Chemical Vapor Deposited (CVD) diamondmaterial, is placed within the amplifier package beneath the GaN MMICs.The metallic coating may, for example, be gold or other electricallyconductive material (e.g., silver, aluminum) or alloys thereof. Themetallic coating may be applied to all sides of the packaging (e.g.,top, bottom and four sides, assuming a square package, although otherpackage shapes can be used as well) of the CVD diamond material with athickness of, for example, about 6-12 micrometers in some suchembodiments. Other thicknesses may also be used as will be appreciatedin light of this disclosure, depending on factors such as the type ofcoating material (or materials) used and the conductivity of thosematerial(s), and desired power levels. The coating material provideselectrical conductivity and facilitates the thermal spreading in thehigh power dissipation environment of the small form-factor package andsignificantly reduces the operating junction temperature of the GaNdevices, which in turn minimizes any performance degradation that mayotherwise be exhibited in GaN devices at high temperature.

The high thermal conductivity heat-spreader material (e.g., 1000 to 1800W/(m° C.), in some example embodiments) may be incorporated, forinstance, into the module flooring to provide improved thermalmanagement for minimizing die junction temperature in the smallform-factor amplifier package. Higher output power levels may beobtained from operation of the amplifier with shorter pulse widths. Insome such example embodiments, providing such higher output power levelsin a form-factor package may be achieved with additional or otherwisesupplemental thermal management techniques beyond convection cooling inorder to maintain the optimal power levels from the GaN MMICs.

High-power-density GaN transistors embedded or otherwise formed intocircuits that operate over broad frequency bandwidths and having thesecircuits integrated into small packages that are capable of managing asevere thermal environment provide a solid-state replacement for TWTAs.The described three elements provide the desired capability and enablessolid-state HPAs that provide comparable performance to TWTAs withoutthe cumbersome size and high-voltage power supply limitations of thetube-based approaches. Example embodiments of the invention havedemonstrated a compact (e.g., 1.2 cubic-inches, although numerous otherform-factors are possible as will be appreciated in light of thisdisclosure) TWTA-replacements for use in an EW frequency band.

Several GaN, High Electron Mobility Transistor (HEMTs) were developed todetermine the optimal or otherwise suitable dual field plate GaN devicetechnology that can be used to address example amplifier applications inthe 1 to 8 GHz frequency band. Using a dedicated mask set incorporatingprimarily discrete device structures, an initial GaN wafer fabricationrun was processed with the specific purpose of evaluating the transistorstructures in an effort to select the optimal configuration of devicetopology, gate-periphery size, and physical device features to selectthe optimal transistor cell for use in MMICs designed to operate in thedesired frequency range. Example devices using three differentgate-lengths (0.25 μm, 0.35 μm, and 0.5 μm) were fabricated to evaluategain-bandwidth performance. The Maximum Stable Gain (MSG) as a functionof frequency is shown in FIG. 1 for three 800 μm GaN HEMT discretedevices of varying gate lengths. While the difference in crossoverfrequency (frequency where the stability factor is above unity) isessentially the same, the gain for the 0.35 μm and the 0.5 μm device is4 dB and 1 dB lower, respectively than the 0.25 μm device. From afabrication point of view, the larger the gate length, the moresimplified the fabrication tolerances and the higher the process yield.

In addition to device gate-length, the device gate-width and criticalchannel dimensions were also evaluated using the structures on thediscrete device mask set. Example transistor structures were includedfor the purposes of investigating gate width variation from 100 μm to500 μm, gate pitch (i.e., gate-to-gate distance) variation from 50 μm to70 μm, and drain-source spacing variation from 2 μm to 4 μm. The resultfrom these studies was the selection of a more optimized transistordevice structure suitable for a baseline GaN device in power amplifierMMIC applications to 10 GHz. From the studies conducted on GaN discretedevices, the selection of a 0.35 μm gate length transistor was chosenfor the desired device baseline in MMIC architectures, as thisgate-length was a good compromise between maximizing fabrication yieldand obtaining adequate gain performance in the amplifier demonstration.A nominal gate pitch of 50 μm and a source-drain channel spacing of 4 μmwere selected as the other desired dimensions for the baseline device.As will be appreciated, such example optimal parameters will vary fromone application to the next, and the present invention is not intendedto be limited to any such example optimal parameters.

The HPA can also be designed to satisfy desired application objectives,which in one example case include: (a) a nominal power gain of 45-50 dB,(b) a nominal output power level of 50 W, and (c) a bandwidthperformance extending from 1-8 GHz. In order to achieve these exampleparameters, a MMIC based amplifier architecture was selected. The MMICapproach was selected for achieving the combination of an 8:1 bandwidthresponse, output power level, and a small overall package size. FIG. 2Aillustrates the block diagram of the amplifier architecture 200 used,which is configured in accordance with one embodiment of the presentinvention. A GaN, MMIC chipset capable of the providing the desiredfrequency performance, gain, and output power was designed to achievethe overall amplifier performance and size objectives. Using modelingextraction techniques from the discrete devices described earlier, acircuit was designed that provided a series of custom GaN amplifierMMICs to use in the HPA module. As can be seen with reference to FIG.2A, the resultant chipset included a pre-driver 202, driver 204, andseveral variants of the HPA 206. The pre-driver 202 and driver 204 MMICswere designed to provide the desired gain and input drive level to theHPA 206, and the HPA 206 was sized appropriately to achieve a nominal 50W output power, in this example embodiment. The pre-driver 202, driver204, and the HPA 206 utilize the prior discussed GaN amplifier MMICdesign. Embodiments are not limited to the described pre-driver 202,driver 204, and the HPA 206 design as will be appreciated in light ofthe disclosure. Various additional stage drivers may be included,substituted, or omitted based on the desired output and other designconsiderations. For instance, in some example embodiment, the HPA 206may be used without the pre-driver 202 and driver 204.

Referring to FIG. 2B, a cross-sectional diagram of the high poweramplifier module 200 is shown which is configured in accordance with oneembodiment of the present invention. As can be seen, in the exampleembodiment the amplification stages are implemented with GaN devices208, which may include the optional pre-driver 202, the optional driver204, and HPA 206 (or a variant thereof, as will be appreciated in lightof this disclosure), and are constructed on top of a substrate 210. Thesubstrate 210 may be constructed using, for example, SiC as previouslydiscussed. Other suitable substrate materials can be used as well. TheGaN, MMIC die or chipsets may each be coupled to the amplifier packagehousing 212 via a metallic coating material 214 of CVD diamond material216. The construction and thicknesses of the coating material 214 andthe CVD diamond material 216 may be, for example, a metal coating of theCVD diamond as described herein. The metallic coating material 214provides electrical conductivity and facilitates the thermal spreadingin the high power dissipation environment of the small form-factorpackage and significantly reduces the operating junction temperature ofthe GaN devices 208. In addition, the diamond 216 provides thermalconductivity to dissipate heat from die 208.

Referring to FIGS. 3A-C, provides example device layout diagrams for thepre-driver 202, driver 204, and the HPA 206, respectively. The cascadeof GaN chips including a single-stage pre-driver 202 MMIC in FIG. 3A, atwo-stage driver 204 MMIC in FIG. 3B, and a 19.2 mm total-gate-peripheryHPA 206 MMIC in FIG. 3C. As can further be seen with reference to FIG.2A, the single-stage pre-driver MMIC 202 incorporates a single-stagearchitecture with a 0.9 mm total gate-periphery stage. The pre-driver202 includes a pre-driver input 302 supplying the amplifying signal topre-driver GaN transistors 304. The output of the pre-driver GaNtransistors 304 is supplied from the pre-driver output 306 forward tothe driver input 310. As can further be seen with reference to FIG. 3A,biasing and impedance matching pre-driver circuitry 308 may also beprovided on the pre-driver chip. The additional circuitry may include,for example, impedance networks, inductive and capacitive decoupling,and/or other signal conditioning circuitry.

The two-stage driver MMIC 204 incorporates a two-stage architecture witha cascade of two 0.9 mm total gate-periphery stages. The amplifyingsignal is feed from the driver input 310 to the driver GaN transistors312 having two stages in series each including three GaN transistors.The output of the driver GaN transistors 312 is supplied from the driveroutput 314 forward to the HPA driver input 316. Similar to thepre-driver 202, additional biasing and impedance matching drivercircuitry 315 may also be provided on the driver chip.

The example detailed embodiment of the HPA 206 shown in FIG. 3C usesNDPA circuit architecture using MMICs. The HPA 206 includes an HPAdriver input 316 supplying the amplifying signal to a HPA driver portion318. The HPA driver portion 318 supplies the amplifying signal to thenon-uniform distributed amplifier strings at the power stage 322. Inthis example case, the power stage 322 includes four strings, herelabeled strings 324A, 324B, 324C and 324D. Referring now to strings 324Aand 324B, the gate trace forms the divider and impedance transformerbetween the output of the HPA driver portion 318 and the input 320 toboth strings 324A and 324B. As will be appreciated in light of thisdisclosure, the gate trace can be a stepped structure that steps theimpedance on the gate electrodes from the first transistor 332A to thelast transistor 332B, such that the associated inductance successivelyincreases from input 320 to output 328. As to the drain electrodes ofthese devices, the inductances formed by drain trace 326 forminductances between the transistors that successively decrease frominput transistor 332A to output transistor 332B. Strings 324A and 324Bhave their outputs combined and share drain trace 326, which functionsto combine the outputs, to inject the drain bias, and to perform animpedance matching function. Strings 324C and 324D have a mirrorstructure and also function to combine outputs with strings 324A and324B. Additional details with respect to the use of NDPA circuitarchitecture in MMICs can be found in the previously incorporated U.S.patent application Ser. No. 11/629,025, entitled, “Solid-StateUltra-Wideband Microwave Power Amplifier Employing Modular Non-UniformDistributed Amplifier Elements.”

FIG. 4 depicts the small-signal gain performance of the exampletwo-stage driver 204 shown in FIG. 3B. As can be seen, the driver 204exhibited a nominal 22 dB of small-signal gain over 2 octaves offrequency (2-8 GHz) when measured on-wafer, and 3 octaves of frequencyperformance when die-attached to a carrier with off-chip biascomponents. The bandwidth improvement was due to the fact that the lowerfrequency response of these MMICs is affected by the inductive loadingof the DC probes used during the on-wafer measurements. When mounted ina housing package with off-chip bypass capacitors, the MMIC responseextends down to 1 GHz, as designed. The single-stage pre-driver 202 MMICwas designed in a similar manner to the driver MMIC 204 using a single0.9 mm total-gate-periphery GaN circuit. The pre-driver 202 achieved 12dB of small-signal gain from 1 to 8 GHz.

FIG. 5 illustrates another example of a non-shared drain HPA 500 thatcan be used in accordance with an embodiment of the present invention.The topology of the non-shared drain HPA 500 is a derivative of adistributed amplifier architecture which, in addition to its inherentwideband frequency performance, also facilities the thermal managementof the module by distributing the heat sources of the devices moreuniformly throughout the MMIC die area. The non-shared drain HPA 500 maybe the same size as example HPA 206 to maintain compatibility in thehigh power amplifier module assembly, as described herein.

The power stage 502 includes two strings, here labeled strings 504A and504B. Similar to the previously described HPA 206, the drain trace 506Aand 506B are each a stepped structure that steps the impedance on thedrain electrodes from the first transistor 512A to the last transistor512B, such that the associated inductance successively decreases frominput 510 to output 508. Strings 504A and 504B have their outputscombined but unlike HPA 206 have individual drain traces 506A and 506B,which functions to combine the cell outputs, to inject the drain bias,and to perform an impedance matching function.

In one specific example case, the non-shared drain HPA 500 may use a 12mm total-gate-periphery output stage with a 4.8 mm total-gate-peripheryfirst-stage. The 2.5:1 drive ratio was used in this example design toprovide sufficient drive power to the second-stage of the non-shareddrain HPA 500 and to maintain the desired level of drive compression.The architecture of this chip utilized six 800 um cell devices in thefirst stage and ten 1.2 mm cell devices in the second stage. The chipsize may be 6.831 mm×4.958 mm.

Referring to FIG. 6, the on-wafer RF measurements of the non-shareddrain HPA 500 is shown when driven 2 dB into compression at a drainvoltage of 36 V and a gate voltage of −1.8 V. An off-chip impedancematching circuit, fabricated on a dielectric substrate was used at theoutput of all the HPA MMICs to impedance-match the MMIC amplifier to 50ohms. The non-shared drain HPA 500 of this example configurationachieved a nominal 40 watts of output power from 1 to 8 GHz when drivenwith 28 dBm input power (as will be appreciated, other test parameterssuch as input power, etc can be used, and the example provided here arenot intended to imply limitations on the claimed invention).

The nearly 34 mm² size of the GaN, MMIC HPA 206 and 500 was relativelylarge; however, the dc-yield of these designs was exceptionally good.Using two test wafers that completed backside processing and having twodifferent buffer materials, the dc-functional yield was examined for thetwo wafers. The yield criteria were a simple pinch-off screen and arelatively benign on-state modulation current screen. Table 1illustrates the results of this analysis. The first wafer was an irondoped buffer wafer and exhibited a dc-yield of 47% (24 out of 51).Another wafer was an aluminum gallium nitride AlGaN doped buffer waferand exhibited a dc-yield of 76% (39 out of 51). Large MMICs that passthis dc-screen have shown excellent correlation with RF functioning diewhen full on-wafer RF evaluation is not practical.

TABLE I DC-Yield of Large-Periphery GaN HPA MMICs from two 3-inch wafersDC Yields Sites/ 07029w2 07029w3 Wafer Periphery Full output D295-4 4 912 4.8/14.4-mm chips D295-5 5 9 13 4.8/9.6-mm  D295-7 9 9 13 4.8/12.0-mmTotal 18 27 38 Half output D295-6 6 12 13 1.6/5.6-mm  chip

Referring to FIG. 7, an HPA module 700 provided a GaN amplifierpackaging that resulted in a module having an RF input 702 and RF output704 with a total volume of 1.2 cubic-inches, in accordance with oneexample embodiment of the present invention. Contained within thisvolume is a cascade of three GaN MMIC chips including a single-stagepre-driver MMIC 202, the two-stage driver MMIC 206, and the 19.2 mmtotal-gate-periphery HPA MMIC 206, as well as bias conditioningcircuitry 706 controlled by DC and control inputs 708. The biasconditioning circuitry 706 may be capable of supporting pulsed gate ordrain bias. The dc conditioning afforded by the bias conditioningcircuitry 706 enabled the application of two bipolar bias voltage ports,one for the negative gate bias and one for the higher-current drainbias. Bias filtering and voltage stability was also addressed by thisbias conditioning circuitry 706. In addition to the off-chip matchingnetwork at the output of the HPA, off-chip bias components were alsoincorporated along each side of the GaN MMICs to facilitate MMICbiasing.

The HPA modules 700 may provide a small form-factor amplifier package.Note that the thermal management strategy employed can also consider thevicinity of the HPA 206 sub-assembly to minimize performance degradationdue to die temperature. This may include, for example, the use of highthermal conductivity materials beneath the GaN HPA MMIC 206, aspreviously described. In this example embodiment, a total of five stagesof amplifier gain are utilized in what is essentially a single transmitchannel amplifier module. The resulting example amplifier provided anominal 45 dB of power gain. The final stage was operated at 3 to 4 dBof gain compression where the output power ranged from 25 to 50 wattsover the 1 to 8 GHz frequency bandwidth.

Referring to FIG. 8A, the measured output power versus frequencyperformance of the HPA module 700 for a 10% duty cycle operation isprovided. Referring to FIG. 8B, the output power versus input power forthree frequencies using a shorter pulse width of 2 μsec is alsoprovided. The peak output power was measured at 50 W at a bias settingof Vd=35 V and V_(g)=−1.9 V under pulsed operation using a 10% pulsedduty cycle and a 10 μsec pulse width. The average output power was 40 Wover the 1 to 8 GHz bandwidth. When evaluated using a more thermallybenign operational mode of a 2 μsec pulse width and 2 msec period, anoutput power of greater than 70 W was measured. This result isconsistent with a nearly 5 W/mm power density from the 14.4 mmtotal-gate-periphery output stage of the HPA MMIC. While presentembodiments have described amplification over 1 to 8 GHz frequency band,it will be apparent in light of the disclosure that embodiment may beused to provide amplification at higher frequency bands up to about 20GHz.

A wideband GaN on SiC amplifier MMIC chip set (pre-driver 202, driver204, and the HPA 206) has been described for use in compact HPA modules700 using small form-factor packages. Using a combination of elementsincluding dual field plate GaN semiconductor technology on siliconcarbide substrate, a non-uniform distributed form of MMIC amplifierarchitecture, and advanced thermal management in the package design, apeak output power of 70 W with an average output power of 40 W wasdemonstrated by this 1.2 in³ HPA amplifier module 700. Operating over a3 octave frequency bandwidth (1 to 8 GHz) with greater than 45 dB ofpower gain, the amplifier module 700 has demonstrated the feasibility ofreplacing tube-based transmitter technology with compact high-powersolid-state technology having reduced size and weight as compared withtube-based amplifiers.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications or additionsmay be made to the described embodiment for performing the same functionof the present invention without deviating therefrom. Therefore, thepresent invention should not be limited to any single embodiment, butrather construed in breadth and scope in accordance with the recitationof the appended claims.

What is claimed is:
 1. A high power amplifier module, comprising: afirst plurality of distributed amplification stages operatively coupledin a first string, wherein a conductive trace of the first string has aninductance that successively decreases from input to output of the firststring; and a second plurality of distributed amplification stagesoperatively coupled in a second string, wherein a conductive trace ofthe second string has an inductance that successively decreases frominput to output of the second string; wherein the high power amplifiermodule is configured to generate an output signal having greater powerthan an input signal.
 2. The high power amplifier module of claim 1,wherein each of the first and second strings comprises gallium nitridetransistor amplification stages interconnected by inductors, and whereinthe values of the inductors are set such that the voltage and currentassociated with one of the gallium nitride transistor stages is equal tothat associated with the other gallium nitride transistor stages tofacilitate maximum power transfer and matching between interconnectedstages.
 3. The high power amplifier module of claim 2, wherein theinductors have values that are set such that the impedance associatedwith one of the gallium nitride transistor gain stage is not equal tothe impedance associated with an adjacent gallium nitride transistorgain stage.
 4. The high power amplifier module of claim 2, wherein theinductors have values that are set such that the impedance associatedwith two of the gallium nitride transistor stages is not equal to theimpedance associated with an adjacent two gallium nitride transistorstages.
 5. The high power amplifier module of claim 1, wherein each ofthe first and second strings comprises gallium nitride transistoramplification stages formed on a silicon carbide substrate.
 6. The highpower amplifier module of claim 1, further comprising a heat spreadermaterial that thermally and electrically couples to the amplificationstages.
 7. The high power amplifier module of claim 6, wherein the heatspreader material is diamond with a metallic coating.
 8. The high poweramplifier module of claim 7, wherein the metallic coating is gold. 9.The high power amplifier module of claim 6, wherein the heat spreadermaterial is chemical vapor deposited diamond with a metallic coating.10. The high power amplifier module of claim 9, wherein the metalliccoating encapsulates the chemical vapor deposited diamond.
 11. The highpower amplifier module of claim 1, wherein the conductive traceassociated with the first string is shared with a third stringcomprising a third plurality of distributed amplification stages. 12.The high power amplifier module of claim 1, wherein the high poweramplifier module has a solid-state package design to accommodate greaterthan 8 W/mm² of power dissipation.
 13. The high power amplifier moduleof claim 1, wherein the high power amplifier module has a nominal powergain of greater than 45 dB, a nominal output power of greater than about50 W, and a frequency bandwidth performance from 1-8 GHz.
 14. The highpower amplifier module of claim 13, wherein the high power amplifiermodule has a package dimension equal or less than 1.2 cubic inches. 15.The high power amplifier module of claim 1, further comprising apre-driver or a driver.
 16. A method for forming a power amplifier,comprising: operatively coupling a first plurality of distributedamplification stages in a first string; providing, as part of the firststring, a conductive trace that has an inductance that successivelydecreases from input to output of the first string; operatively couplinga second plurality of distributed amplification stages in a secondstring; providing, as part of the second string, a conductive trace thathas an inductance that successively decreases from input to output ofthe second string; and providing a heat spreader material that thermallyand electrically couples to the amplification stages.
 17. A high poweramplifier, formed by the method of claim 16, wherein each of the firstand second strings comprises gallium nitride transistor amplificationstages.
 18. The high power amplifier of claim 17, wherein at least oneof the gallium nitride transistor amplification stages comprises a dualfield plate transistor with a silicon carbide substrate.
 19. The highpower amplifier of claim 17, wherein values of the associated inductanceare set such that the voltage and current associated with one of thegallium nitride transistor amplification stages is equal to thatassociated with the other gallium nitride transistor amplificationstages to facilitate maximum power transfer and matching betweeninterconnected stages.
 20. The high power amplifier of claim 17, whereinthe heat spreader material is chemical vapor deposited diamond with ametallic coating.
 21. The method of claim 16, wherein the conductivetrace associated with the first string is shared with a third stringcomprising a third plurality of distributed amplification stages.